1. Field of the Invention
The present invention relates to an image pick-up device for picking up a manuscript image.
2. Description of Related Art
When an image of a color manuscript is picked up, the light reflected from the manuscript is focused on a CCD sensor through a lens, and the light is then converted into an electric signal. This electric signal is further converted into digital image data to be outputted from a printer or the like. As shown in FIG. 1, when a white light goes through a lens, depending on the difference of the wave length of the light components, the longer the wave length of the light is, the more inside it condenses, while the shorter the wave length of the light is, the more outside it condenses, which is more so at the edge of the lens compared to at the central part of it. Therefore, when an image like a vertical line is condensed on the CCD sensor, the phases of R(red), G(green) and B(blue) may be shifted from each other. In this manner, the light is influenced by the chromatic aberration of the lens to cause a color shift at the edge of the CCD sensor in the main scanning direction.
Chromatic aberration is not a problem with an image of a comparatively smooth density profile such as that of color patches. It causes, however, a color shift at the edge of a letter or the like. Especially, at the edge of a black letter, a mistaken determination causes a color bleeding or a separation of the letter around the periphery of the letter. Therefore, high quality lenses are required for a copier using a color CCD sensor. However, even if the lens function was improved, the lens system would be larger, which ends up with a larger image pick-up device including the optical system. And the lack of uniformity among the lens part products cannot also be ignored. Therefore, it is necessary to correct this chromatic aberration in the image processing system.
In picking up an image, the main method for chromatic aberration correction is to mix adjoining image data by using chromatic aberration interpolation factors. As the chromatic aberration correction circuit shows in FIG. 2, for example, with respect to R and B data picked up through each scanning of a manuscript, a plurality of phase correction circuits perform a chromatic aberration operation for adjoining three pixels by using plural combinations of phase interpolation factors calculated when the lens(es) are(is) designed, and calculate color saturation data (the maximum value-the minimum value of R, G, and B) for each combination of respective colors to select R, G and B data that minimize this value of color saturation. In this way, chromatic aberration is corrected through calculation. Accordingly, whatever the image data are, the ones with the minimum color saturation are selected. The effect of a chromatic aberration stands out at the edge of black letters but not in other places, therefore, this correction was believed to be enough in practical use.
This color correction operation, however, doesn't have a good correction precision because the interpolation factors are fixed. The interpolation factors can only be selected within limits and moreover, for some image data, different data from the actual chromatic aberration could possibly be selected. And, because the interpolation factors depend on the pick-up resolution or the lens system, in case the pick-up resolution or the lens system is changed, it would be necessary to change the whole design accordingly, which means it lacks the flexibility for general purposes. For general purposes, the interpolation factors should be variable depending on the resolution or the like.